mdb_vread() and mdb_vwrite()

These functions
provide the ability to read and write data from a given target virtual address,
specified by the addr parameter. The mdb_vread() function returns nbytes for
success, or -1 for error; if a read is truncated because only a portion of
the data can be read from the specified address, -1 is returned. The mdb_vwrite() function returns the number of bytes actually written
upon success; -1 is returned upon error.